man-pages/man2/sendfile.2

.\" This man page is Copyright (C) 1998 Pawel Krawczyk.
.\" Permission is granted to distribute possibly modified copies
.\" of this page provided the header is included verbatim,
.\" and in case of nontrivial modification author and date
.\" of the modification is added to the header.
.\" $Id: sendfile.2,v 1.5 1999/05/18 11:54:11 freitag Exp $
.\" 2000-11-19 bert hubert <ahu@ds9a.nl>: in_fd cannot be socket
.\"
.\" 2004-12-17, mtk
.\"	updated description of in_fd and out_fd for 2.6
.\"	Various wording and formatting changes
.\"
.\" 2005-03-31 Martin Pool <mbp@sourcefrog.net> mmap() improvements
.\"
.TH SENDFILE 2 2004-12-17 "Linux" "Linux Programmer's Manual"
.SH NAME
sendfile \- transfer data between file descriptors
.SH SYNOPSIS
.B #include <sys/sendfile.h>
.sp
.BI "ssize_t sendfile(int" " out_fd" ", int" " in_fd" ", off_t *" \
                      offset ", size_t" " count" );
.\" The below is too ugly. Comments about glibc versions belong
.\" in the notes, not in the header.
.\"
.\" .B #include <features.h>
.\" .br
.\" .B #if (__GLIBC__==2 && __GLIBC_MINOR__>=1) || __GLIBC__>2
.\" .br
.\" .B #include <sys/sendfile.h>
.\" .br
.\" #else
.\" .br
.\" .B #include <sys/types.h>
.\" .br
.\" .B /* No system prototype before glibc 2.1. */
.\" .br
.\" .BI "ssize_t sendfile(int" " out_fd" ", int" " in_fd" ", off_t *" \
.\"                       offset ", size_t" " count" )
.\" .br
.\" .B #endif
.\"
.SH DESCRIPTION
.BR sendfile ()
copies data between one file descriptor and another.
Because this copying is done within the kernel,
.BR sendfile ()
is more efficient than the combination of
.BR read (2)
and
.BR write (2),
which would require transferring data to and from user space.

.I in_fd
should be a file descriptor opened for reading and
.I out_fd
should be a descriptor opened for writing.

If
.I offset
is not NULL, then it points
to a variable holding the file offset from which
.BR sendfile ()
will start reading data from
.IR in_fd .
When
.BR sendfile ()
returns, this variable
will be set to the offset of the byte following the last byte that was read.
If
.I offset
is not NULL, then
.BR sendfile ()
does not modify the current file offset of
.IR in_fd ;
otherwise the current file offset is adjusted to reflect
the number of bytes read from
.IR in_fd .

.I count
is the number of bytes to copy between the file descriptors.

Presently (Linux 2.6.9):
.IR in_fd ,
must correspond to a file which supports
.BR mmap (2)-like
operations
(i.e., it cannot be a socket);
and
.I out_fd
must refer to a socket.

Applications may wish to fall back to
.BR read (2)/ write (2)
in the case where
.BR sendfile ()
fails with
.B EINVAL
or
.BR ENOSYS .
.SH "RETURN VALUE"
If the transfer was successful, the number of bytes written to
.I out_fd
is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EAGAIN
Non-blocking I/O has been selected using
.B O_NONBLOCK
and the write would block.
.TP
.B EBADF
The input file was not opened for reading or the output file
was not opened for writing.
.TP
.B EFAULT
Bad address.
.TP
.B EINVAL
Descriptor is not valid or locked, or an
.BR mmap (2)-like
operation is not available for
.IR in_fd .
.TP
.B EIO
Unspecified error while reading from
.IR in_fd .
.TP
.B ENOMEM
Insufficient memory to read from
.IR in_fd .
.SH VERSIONS
.BR sendfile ()
is a new feature in Linux 2.2.
The include file
.I <sys/sendfile.h>
is present since glibc 2.1.
.SH "CONFORMING TO"
Not specified in POSIX.1-2001, or other standards.

Other Unix systems implement
.BR sendfile ()
with different semantics and prototypes.
It should not be used in portable programs.
.SH NOTES
If you plan to use
.BR sendfile ()
for sending files to a TCP socket, but need
to send some header data in front of the file contents, you will find
it useful to employ the
.B TCP_CORK
option, described in
.BR tcp (7),
to minimize the number of packets and to tune performance.

In Linux 2.4 and earlier,
.I out_fd
could refer to a regular file, and
.BR sendfile ()
changed the current offset of that file.
.SH "SEE ALSO"
.BR open (2),
.BR mmap (2),
.BR socket (2),
.BR splice (2)